1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the same, which are suitable for use in a television camera for broadcasting, a video camera, a digital still camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, there has been a demand for a zoom lens system having a large aperture ratio, a high zoom ratio, a wide field angle, and high optical performance for image pickup apparatuses such as a television camera, a silver-halide film camera, a digital camera, and a video camera. Of the image pickup apparatuses, it has been demanded that an image pickup apparatus for a broadcast color television camera have a zoom lens system having a long back focus because a color separation optical system or various filters are provided in front of (on object side of) an image pickup unit. A positive lead type four-unit zoom lens system has been known as a zoom lens system satisfying the demand.
The four-unit zoom lens system includes, in order from the object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. The first lens unit includes a focusing lens unit. The second lens unit is used for magnification-varying. The third lens unit moves during magnification-varying.
In the case of the four-unit zoom lens system described above, in order to obtain a large aperture ratio, a high magnification-varying ratio, a shooting field angle 2ω at a wide angle end which is equal to or larger than 78°, and high optical performance over the entire magnification-varying range, it is necessary to suitably set the refractive powers of the respective lens units and the lens structures.
For example, when the lens structure of the fourth lens unit for imaging is not suitably set, it is difficult to reduce variations in chromatic aberration, spherical aberration, halo, coma, and chromatic difference in spherical aberration and achieve a high zoom ratio and high performance while maintaining a predetermined back focus.
In particular, when a lens unit (extender lens) for changing a focal length range of the entire system is provided so as to be removably inserted between the lens subunit located on the object side and the lens subunit located on the image side onto the optical path in the fourth lens unit, it is important to suitably set the lens structure of the lens subunit located on the image side in the fourth lens unit.